< LASS II 



293 



In tin- earliest shell -ro\\tli >tap-s Ilrachiopod shells are invariably smooth, 

 and may remain so throughout lift 1 , but tliu greater number develop radial 

 striae, ribs, or undulations, and these arc usually crossed by concentric growth 

 lines, or lamellar, which arc sometimes of great width, or may IK- extended into 

 spines. There may be more or less long tubular spines scattered over one or 

 both valves, or sometimes restricted to a .-inude row along the cardinal line, 

 t'nder the term Lnri,-<it,n-, Leopold von Buch included all Brachiopods in which 

 the radial folds, or costae. are arranged in regular succession in such manner 

 that elevated ridges at the anterior margin of one valve coincide with the 

 indentations of the other. In the Biplicatac, a median fold or sinus is 

 bordered on either side by a broad fold. In the Cinctae, the plications of the 

 two valves meet at the anterior margin in such manner as to form a straight 

 instead of a crenulated line, as in the Biplicatae. In young specimens the 

 ribs and folds are less prominent and numerous than at maturity. In very old 

 or senile individuals the shell is usually thickened and obese, and the growth 

 lines are much crowded anteriorly. At this stage old specific characters are 

 seen to disappear, and at the same time new ones may be introduced. 



Shell xtni<inre. The test of Brachiopods is composed of laminae of various 

 structure and composition, but differs considerably from that of Molluscs. 

 The shell may 

 be wholly cal- 

 careous or alter- 

 nately calcareous 

 and corneous. 

 When entirely 

 calcareous the 

 laminae are 

 never more than 

 three in number : 

 an inner thick 

 prismatic layer, 

 an intermediate 

 laminar layer, 

 and an outer 

 epidermal film. 



Thp innftr lavpr 

 ''Iyer 



is made up of 



flattened prisms of calcite arranged parallel to one another with great 

 regularity, and forming an acute angle with the surface of the shell (Fig. 

 489). In the Thecidiidae these fibrous prisms are so intimately united with 

 one another that the shell substance appears almost homogeneous. Very 

 often the fibrous layer is perforated by a series of minute canals which pass 

 from one surface of the valve to the other in a more or less vertical direction, 

 and are somewhat dilated externally. These canals contain tubuli, or certain 

 prolongations derived from the mantle, but never communicate with the 

 exterior, owing to the fact that the laminar layer of the shell is always 

 covered with a chitinous epidermis (])en^'acmn). With the aid of a 

 magnifier the openings of these canals are visible in fossil forms, and they 

 may be also seen in recent specimens after the epidermis has been removed 

 by an application of caustic potash (Fig. 490). According to the presence 



Fio. 489. 



Prismatic fibrous struc- 

 tureof shell of Rhynchonella 



(Hemithiir!*) y.x/M-.-m, itni/, 

 (after Carpenter). 



A, Punctate exterior of a Terebratuloid shell, slightly 

 enlarged, li, Vertical section of shell of M</cll<',<i<i 

 favescens, showing distally cnlar^-d tubules, 1(l "/]. 

 C, Inner surface of Mifii,n.;<i shell showing ends nf 

 tubules and oblique calcareous prisms, iffy (after 

 Carpenter). 



